Why, when someone we trusted turns against us, do we say he’s “shown his true colours”?

Sailing under false colours means to sail under the enemy flag, and it was once a legitimate naval manoeuvre used to get close enough to the enemy for a surprise attack. At the last moment, just before opening fire, the false colours were lowered and replaced by the ship’s “true colours.” Although such deception is now considered dishonourable, we still say when someone we trusted reveals himself as the enemy that he is showing his “true colours.”